Ground level solar energy radiation is so dilute that devices which concentrate sunlight are utilized before converting it to electrical or thermal energy useful for a wide range of applications. These devices typically use geometric optics in the form of mirrors and lenses. In doing so there are cost issues and the requirement of tracking. Optics with high concentration capabilities need to be tracked or be directed towards the sun on an hourly basis while even moderate concentration devices require seasonal adjustments to cope with seasonal changes.
Traditional solar energy conversion is achieved by flat plate technology, in which solar radiation directly impinges upon a large array of photovoltaic cells typically made of high purity Silicon. Conventional photovoltaic cells are formed from monocrystalline, polycrystalline, amorphous silicon. Because the cost of photovoltaic cells and the demand for semiconductor material are both high, the cost of the high volume of semiconductors required for conventional photovoltaic is a deterrent to widespread use. Further, typical efficiencies of the Silicon solar cells are moderate in the range of 15-20%. In contrast, concentrated photovoltaic systems increase the efficiency by converting solar energy to “direct current” electricity by using mirrors/lenses to focus the sunlight onto a high efficiency small and thus much less expensive solar cell. Hence, light concentrators can reduce the cost of electricity generated using solar photovoltaic by reducing the amount of high cost semiconductor material needed for photovoltaic conversion.
In the last three decades, several solar concentrator designs based on geometric and luminescent concentrators have been proposed and developed. Geometric designs are based on reflective, refractive, fibre optic, holographic systems in which light is manipulated/redirected onto a receiver where a photovoltaic (PV) cell converts the light energy into electrical energy. On the other hand, luminescent concentrators typically use absorptive dyes/pigments where the light energy is absorbed and re-emitted at a large (red-shifted) wavelength. This emitted light is then guided in the substrate through total internal reflection towards the edges. A PV cell placed at the edges of the substrate converts the light energy into electrical energy.
Several Luminescent concentrator designs have been proposed and realized. In the late 70's a novel type of collector was extensively investigated consisting of a transparent sheet doped with appropriate organic dyes by W.H. Weber et al Appl. Opt., 15, 2299, (1976). U.S. Ser. No. 12/414,722 disclose a radiation concentrator suitable for use in concentrating solar radiation for efficient and low cost solar photovoltaic use, especially in window-mounted devices. U.S. Pat. No. 4,227,939 propose a new type of solar collector, the Luminescent Solar Concentrator (LSC) based on light-pipe trapping of molecular fluorescence. The main advantage of luminescent concentrator designs compared to the geometric concentrator designs is that they do not need precise tracking. However, luminescent concentrators typically suffer from self-absorption problem, low absorption bandwidth; hence typically a stack of LSC's are required to harness the whole solar spectrum, isotropic emission; only a fraction of light is guided in the substrate and contributes to light concentrations and long term degradation problem especially under high light concentrations. These entire factors give rise to low concentrations (typically close to 10×) compared to the geometric concentrator designs.
U.S. Pat. No. 6,476,312 discloses a radiation concentrator for use with a photo voltaic device. The device uses a distribution of quantum dots instead of organic dyes/pigments in-order to red-shift the incoming radiation. The red-shifted radiation is then internally guided in the waveguide which is converted to electrical energy using a photovoltaic device attached to the end of the waveguide. The advantage of this approach compared to organic dyes/pigments is higher absorption efficiency but this design require a large distribution of quantum dots to minimize the overlap between the emission and the absorption spectrum in which case absorption spectrum also becomes broader. Further, the quantum dots have to pack extremely close to each other to achieve large light collection efficiency. This will result in formation of clusters affecting the desired red shifts and reduction of light concentration efficiency. Further, quantum dots also give isotropic luminescence and hence only a fraction of light is coupled into the guiding substrate.
In geometric concentrator designs, the concentrating optical elements typically have a non-zero focal length making the concentrated photovoltaic modules bulkier than the non-concentrating counterparts. Concentrated photovoltaic systems also need precise tracking to maximize energy conversion efficiency and this bulkiness is disadvantageous not only for tracking but also in terms of handling, shipping and material costs. It is possible to obtain less bulky concentrated photovoltaic cell module while maintaining the same concentration factor by reducing the size of the photovoltaic cell. Also the concentrators typically require a cooling system which further adds to cost, size and inefficiency of the solar photovoltaic system. Many methods/arrangements have been tried to reduce the size and increase the efficiency of the concentrator in the prior art by developing planar concentrator designs.
U.S. Ser. No. 12/231,046 discusses a family of flat concentrator PV panels wherein an array of enhanced light beam splitters coupling with a plurality of optical grooves efficiently collects light and transmits collected light substantially to the active surface(s) of an array of size-reduced PV cells with low aspect ratio. However, the concentrations achieved using these designs are not very high and the performance of the concentrator may suffer at various angles of incidence.
In “High Efficiency Solar Cells Enhanced with Diffraction Grating” by Sin-inchi Mizuno et al., Technical digest of the International PVSEC-11, Sapporo, Hokkaido, Japan, 341 (1999), diffraction gratings are secured onto the front and back surfaces of thin film, single crystal silicon using an adhesive material having refractive index of 2, such that the average light path of thin film silicon was tripled when compared with silicon thin films having a mirror placed on the back surface thereof. As Silicon has low absorption coefficient compared to other semiconductors, these designs are typically employed to increase the absorption length of light in the solar cell there by increasing the efficiency of photovoltaic conversion. These designs are most effective to increase the absorption of light near the band gap energy of the Silicon
U.S. Ser. No. 12/113,705 discuss a planar light-guide solar panel for harvesting and trapping the light inside of a dielectric material. The surface of the light-guide solar panel consists of a light-insertion stage where in light is guided into the substrate using parabolic/elliptical reflectors, Winston cone optics, Cassegrain optics etc. To maximize the light guiding into the substrate the profile of the light guiding optics, light insertion points and substrate taper etc. need to be carefully optimized.
However the common problem faced with all of the above designs during reduction in size is the requirement of skilled assembling of primary and secondary mirrors and specialized tooling and the accurate placement of solar cells
Components which are designed in such a way to simplify the assembly process would greatly improve the chances of a solar energy system to be successful. Additional considerations such as ease of installation, serviceability and durability against environmental; conditions are also important. Further, due to large light concentrations involved, all the IR light below the band gap of the solar cell goes unutilized in electricity conversion and causes excessive heating of the solar cell and hence these designs typically require active cooling.